True 3D video displays target physical duplication of volume filling light of a scene. If a perfect duplication can be achieved, any observer, human or not, interacting with the duplicate light will see an exact ghost-like duplicate of the original scene. Most of the techniques commonly used today for 3D video are based on stereoscopic technique, as in commercial 3D movies. Even though multi-view video techniques are better than stereoscopy, these techniques are still far from ideal true 3D. The only true 3D technique is the holography. An overview of different 3D display techniques can be found in P. Benzie, J. Watson, P. Surman, I. Rakkolainen, K. Hopf; H. Urey, V. Sainov and C. von Kopylow, “A Survey of 3DTV Displays: Techniques and Technologies”, IEEE Tran. on Circuits and Systems for Video Technology, vol 17, no 11, pp 1647-1658, November 2007. A survey on existing holographic displays can be found in F. Yaras, H. Kang and L. Onural, “State of the Art in Holographic Displays: A Survey”, J. of Display Technology, vol 6, no 10, pp 443-454, October 2010.
One of the major problems of the state-of-the art electro-holographic video cameras or displays is the required high spatial bandwidth (resolution) of the underlying electro-optical device. Holographic patterns are typically complicated fringe patterns where the fringes have very fine details; the detail is in the order of the wavelength and therefore, is in the micrometer range for optical holography. In addition to the high spatial bandwidth, a rather large size, ranging from a few square centimeters to many square decimeters, is needed for a satisfactory viewing experience. Such a fine resolution over a rather large area means a very large space-bandwidth product. Therefore, for a digital display, the device on which the holograms are electronically written must have very small pixel sizes (in the order of micrometers) that in turn, brings the number of such pixels to the order millions per square millimeter of the device. The same is also true for the camera: the electro-optical capturing device that captures the holographic fringe pattern must have a high spatial bandwidth (resolution) and a very large space-bandwidth product. The difficulty in the design and manufacturing of such high-resolution display or capture devices is one of the main obstacles that prevent consumer quality holographic video displays.
The effect of the resolution on the display is one of the important issues to consider and it can be analyzed as given in L. Onural, F. Yaras and H. Kang, “Digital Holographic Three-Dimensional Video Displays”, Proc. of the IEEE, vol 99, no 4, pp 576-589, April 2011. Simply, each small patch on the display device is a local diffractor that distributes the incident light to different directions as it passes (transmissive case) or reflects (reflective case) from that patch. The resolution is directly related to the diffraction angle: larger resolutions result in finer patterns which in turn result in larger diffraction angles; therefore, a large resolution is needed to distribute outgoing light within a larger angle. The size of this angle also determines the viewing angle of the observer. The same is also true for capture devices: larger angles of incidence results in high resolution fringes to capture. Therefore, the resolution, both at the display or the capture, is directly related to viewing and capturing angles.
A parabolic mirror based image capture and display was disclosed in U.S. Pat. No. 4,229,761 in 1980. In that patent what the camera captures, after reflection from a parabolic mirror, is still the conventional video (intensity of light after it passes through a lens is captured), and not the fringe patterns as in a holographic capture device. The information carried by the captured intensity of light is not sufficient for a successful 3D operation. The system disclosed in U.S. Pat. No. 4,229,761 yields 2D intensity images floating in the air as a real image from a parabolic mirror. These floating 2D images may be well-focused, or may contain out-of-focus blurred areas, depending on the aperture of the camera as in conventional photography/video. The device here in this invention also has a concave mirror and an image capture device at the camera side, and a concave mirror and a display device at the display side, but both the capture and display techniques are different (holographic versus conventional video) and the relative location and the geometry of the capture and the display devices with respect to the mirror are different. Here in this invention the holographic capture and display devices cover a large area of the aperture of the paraboloid mirror, where in the camera and projection devices in U.S. Pat. No. 4,229,761 have conventional smaller size lens apertures.
A projection device having a coherent light beam-generator that generates a light beam and a beam expander disposed to receive the light beam and to emit an expanded light beam was disclosed in U.S. Pat. No. 7,738,151. The projection device disclosed in U.S. Pat. No. 7,738,151 also includes a digital micro-mirror device disposed to receive a holographic transform of an original image and to display the holographic transform for illumination by the expanded light beam into a holographic light beam with a convergent or focusing lens disposed to receive and modulate the holographic light beam and a liquid crystal plate volumetric image reconstructor that receives the focused holographic light beam and emits a 3-dimensional holographic image of the original image. The projection device disclosed in U.S. Pat. No. 7,738,151 is capable of generating true 3D images of only the synthetic (computer-generated) 3D scenes. Said projection device needs “image reconstructor”, and therefore, does not attempt to replicate the physical volume filling light distribution. Instead of that, what is achieved by the invention in U.S. Pat. No. 7,738,151 is commonly called as “volumetric display” in the literature. Therefore, although the U.S. Pat. No. 7,738,151 discloses a holographic projector, the scope of the invention disclosed in that document is different than the invention disclosed in detailed description part of this document.